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OBJECTIVE: The adalimumab originator Humira® introduced a new citrate-free formulation in 2016, before the patent expiry that occurred in the European Union in October 2018. Some of the adalimumab biosimilars that were subsequently marketed are citrate-free, while others are not. Since citrate as an excipient is associated with pain at the injection site, recent anecdotical reporting in Italy raised the issue of possible prescription biases related to the differences in formulation existing among the various adalimumab products. In this study, we analyzed the data obtained from the ‘Rete Nazionale di Farmacovigilanza’ (Pharmacovigilance National Network) to investigate whether, and to what extent, the differences in the formulation of the various adalimumab versions had an impact on the rate of injection site reactions reported in Italy in the period 2016-2019. MATERIALS AND METHODS: A search was conducted based on 3 search criteria: (1) time frame; (2) suspected drugs, and (3) adverse reaction type. Reports classified in the System Organ Class “Administration site conditions” were analyzed by year, product, and type of adverse event (whether including or not ‘pain’). Data were reported both as absolute numbers, as well as signaling rates, considering the consumption data expressed as defined daily doses (DDD). RESULTS: We found that: (1) The change in Humira® formulation introduced in august 2016 was followed by a decrease in the reports of injection site reactions (from 45 in 2016 to 12, 12 and 8 in 2017, 2018, and 2019, respectively); (2) after the introduction of biosimilars during 2018, in 2019 a marked shift in reporting toward biosimilars was observed (52 out of 60; 87%). CONCLUSIONS: While the decrease in Humira® reports is consistent with the improved tolerability of the new formulation, the huge increase in biosimilar reporting may be only in part explained by the differences in formulation and cannot be accounted for by a parallel increase in exposure, since 58.3% of total DDDs provided in 2019 were still attributed to Humira®. [ABSTRACT FROM AUTHOR]